Covalent organic framework-based heterojunction photocatalysts.

Photocatalysis, as a sustainable, green and low-cost technology that converts solar energy into chemical energy, holds great potential to address the issues of environmental pollution and energy shortages. However, the rapid charge recombination in semiconductors limits their potential for achieving high photocatalytic activity. Heterojunctions, including inorganic-inorganic, organic-inorganic, and organic-organic heterojunctions, have proven to be an effective strategy for inhibiting charge recombination. By virtue of the high crystallinity, tunable structure and accessible pore systems of covalent organic frameworks (COFs), COF-based heterojunction photocatalysts have shown appealing prospects for promoting solar-to-energy conversion by improving charge separation, enhancing light-harvesting ability and facilitating photoredox reactions. In this review, the state-of-the-art progress in COF-based heterojunction photocatalysts including their mechanism, classification, synthetic strategies and applications are summarized. In particular, the synergistic effect and charge transfer mechanism as well as the structure-activity relationships of COF-based heterojunctions are thoroughly reviewed. Finally, the challenges and outlooks of COF-based heterojunction photocatalysts are also discussed. It is believed that this review can stimulate inspiration for the future development of efficient COF-based heterojunction photocatalysts.